This is a nationalization of PCT/SE00/01621 filed Aug. 23, 2000 and published in English.
The present invention relates to microwave radio frequency transmission line circuits generally and more specifically to four port hybrids.
The requirement to integrate as much as possible in even smaller volumes calls for the study and development of new types of hybrids.
Hybrids are per se well known and well understood in this art in its waveguide, coaxial, microstrip and stripline forms. Typical prior art hybrids are branch directional coupler, Lange coupler and tandem coupler. These hybrids are fundamentally four port devices that accept a signal at an input port, divide the signal in half internally and then supply the divided signal to two output ports. In an ideal quadrature hybrid, the difference in phase angle between the output ports remains at 90 degrees and the amplitude of the output signals remain equal across the useful bandwidth of the device. There is essentially no output from the fourth port as it is isolated from the input port, and in many instances said port is terminated internally. Once the input port is selected the others are defined automatically.
The most common hybrid structure is a branch directional coupler. The problem with said hybrid is too large to be of any interest at a frequency band used in mobile telephones, e.g. a GSM or a PCS frequency band.
Another hybrid is the one based on coupled lines arranged on one side of a dielectric substrate. The problem with said hybrid is that it cannot be realised using standard PCB technology due to too narrow gap between.
Yet another hybrid is the one based on coupled lines arranged on opposite sides of a dielectric substrate. The problem with said hybrid is that the physical dimensions are too large and the necessity to use both sides of said substrate with the added problem of double sided alignment.
A further hybrid is the so called Lange coupler. The problem with said hybrid is that the required 3 dB coupling between the transmission lines has to be done with narrow transmission lines which are too narrow to be cross connected by commercially available PCB (Printed Circuit Board)-jumpers. Another problem with the lange coupler is that the physical dimension is too large to be of any interest in applications demanding small is space.
Still another hybrid is the so called tandem coupler. The problem with said hybrid is that the physical dimension is too large.
It is an object of the present invention to provide a four port hybrid which overcomes or at least reduces the above mentioned problems.
Another object of the present invention is to provide a hybrid with comparably small physical dimensions and improved electrical parameters.
According to the present invention there is provided a four port hybrid as claimed in claim 1.
One advantage with the present invention is that the hybrid can be manufactured in stripline or microstrip with comparably wide strips and comparably wide gaps between said strips that results in a high Q-factor of the transmission lines which in turn leads to small insertion loss.
Another advantage with the present invention is that the hybrid is less sensitive to fabrication tolerances and by that is inexpensive to manufacture.
Yet another advantage is that the present invention being small enough to make an implementation in MMIC (Monolithic Microwave Integrated circuit) technology possible.
Still another advantage is that the present invention has improved both reflection and insertion loss compared to already existing hybrids.